25-2 The Diffraction Grating

Answer to Essential Question 25.1 (a) The only points for which
changing the wavelength has no impact on the interference of the waves
are points for which the path-length difference is zero. Thus, the point in
question must lie on the perpendicular bisector of the line joining the
sources. (b) If we re-arrange Equation 25.3 to solve for the sine of the
angle, we get
.
Thus, decreasing the wavelength decreases sin θ, so the pattern gets
tighter. Decreasing d, the distance between the sources, has the opposite
effect, with the pattern spreading out. We can understand the wavelength
effect conceptually in that, when the wavelength decreases, we don’t
have to go as far from the perpendicular bisector to locate points that are
half a wavelength (or a full wavelength) farther from one source than the
other. Figure 25.3 shows the effect of decreasing the wavelength, or of
decreasing the distance between the sources.
Figure 25.3: The top diagram shows the interference pattern produced
by two sources. The middle diagram shows the effect of decreasing the
wavelength of the waves produced by the sources, while the bottom
diagram shows the effect of decreasing the distance between the sources.
25-2 The Diffraction Grating
Now that we understand what happens when we have two sources emitting waves that interfere,
let’s see if we can understand what happens when we add additional sources. The distance d
between neighboring sources is the same as the distance between the original two sources, and the
sources are arranged in a line. All the sources emit identical waves that are in phase.
EXPLORATION 25.2A – Adding sources
Step 1 – Consider a point a long way from two
sources. The sources are a distance d apart. The
point is one wavelength farther from one source
than the other, so constructive interference
occurs at the point. When we add a third source,
so that we have three sources equally spaced in a
line, separated by d, do we still get constructive
interference taking place at the point?
Yes. As the diagram in Figure 25.4 shows, the
path-length difference for the third source and the
source it was placed closest to will also be one
wavelength. Now we get constructive interference
for three waves at once, not just two, so the amplitude of the
resultant wave is larger than it was with only two sources.
Figure 25.4: For a point that is one
wavelength farther from one source
than another, adding a third source
results in even larger amplitude
because of constructive interference.
Step 2 – If we consider a different point that is half a
wavelength farther from one of two sources than the
other, destructive interference occurs at the point. When we add a third source, so that we have
three sources equally spaced in a line, separated by d, do we still get destructive interference
taking place at the point?
Chapter 25 – Interference and Diffraction
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No. The destructive interference at the point was caused by the cancellation between the waves
from the first two sources. Adding a third source does not change the fact that the first two waves
cancel one another, so there is nothing to cancel the third wave.
Step 3 – For three sources, what path-length difference (between zero and one wavelength)
between neighboring sources results in completely destructive interference? With three sources,
it turns out that there are two path-length differences between 0 and one wavelength that result in
completely destructive interference, these being one-third and two-thirds of a wavelength.
Step 4 – What if we have N sources, where N is any integer greater than 1. Is there a general
rule for predicting the angles at which constructive interference occurs? What about
destructive interference? Constructive interference occurs at the same points for N sources that it
does for 2 sources, so the equation
still applies for situations with N > 1 sources.
There are N – 1 places where destructive interference happens in between each interference
maximum, so we generally dispense with an equation for destructive interference when N > 2.
Key idea: The equation
applies to any number of sources > 1, as long as the
sources are equally spaced. With multiple sources, it is much easier to produce destructive
interference than it is to produce completely constructive interference, so there is no simple
equation for destructive interference.
Related End-of-Chapter Exercises: 7, 16 – 18, 38, 39, 48.
The Diffraction Grating
A diffraction grating is essentially a large number of equally
spaced sources, and thus the
equation applies.
One application of diffraction gratings is in spectroscopy,
which involves separating light into its different wavelengths,
a process that astronomers, or chemists, can use to determine
the chemical makeup of the source producing the light. In
actuality, a diffraction grating is typically a glass or plastic
slide with a large number of slits (long thin openings between
long thin lines). A diffraction grating (which should probably
have been named an interference grating) offers two main
advantages over a double slit. First, the more openings the
light passes through, the brighter the interference maxima
are. Second, the more openings there are, the narrower the
bright lines are in the interference pattern, which is important
when trying to resolve two similar wavelengths. Figure 25.5
shows the increased sharpness that results from adding slits.
Figure 25.5: Adding sources (or slits that
light goes through) results in sharper
interference maxima. Each case shows the
relative intensity at various points. The
amplitude of the peaks also grows as
sources are added.
EXPLORATION 25.2B – Double-slit geometry
When light of a single wavelength (say, from a laser) is incident on a double slit (or a diffraction
grating, which gives a sharper pattern), we get a pattern of bright and dark fringes on a screen
beyond the double slit. Such a pattern is shown in Figure 25.6. The bright fringes come from
constructive interference, and the dark fringes come from destructive interference.
Chapter 25 – Interference and Diffraction
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Step 1 – If we wanted to increase the distance
between the bright spots on the screen, what would
we change about d or λ?
This should be something of a review. If we rearrange the equation for constructive interference, we
get:
mλ
sin θ =
.
d
Increasing the value of sinθ will increase the value of
θ, which means that the lines of constructive
interference will be further apart, in terms of the
angles between them. This will spread out the pattern
on the screen. To increase sinθ, we can either replace
the first laser with a laser that emits light of a longer
wavelength (switch from blue to red, for instance), or
we can switch to a double slit that has a smaller value
of d. These two options are illustrated in Figure 25.7.
Figure 25.6: The geometry of the double-slit pattern, for blue light. The top shows an overview
of the interference pattern. The bottom has the pattern of bright and dark fringes on the screen. In
between is a graph of the intensity of the fringes in the pattern, as a function of position.
Figure 25.7: The pattern on the left shows how the situation of Figure 25.6 changes if we
change to a larger wavelength, while the one on the left shows the change of switching to a
smaller d.
Chapter 25 – Interference and Diffraction
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Step 2 – What if we only have one laser and one
double slit, so we can’t change the wavelength or d. Is
there a different way to increase the distance between
the bright spots on the screen?
Yes. If we move the screen farther from the double slit,
the screen will intercept the light from the grating after
the bright lines in the pattern have been able to spread
out farther, increasing the distance between the bright
spots on the screen. This is illustrated in Figure 25.8.
Step 3 – As shown in Figure 25.9, let’s use L to denote
the distance from the double slit to the screen, and ym
to denote the distance from the central bright spot on
the screen to the mth bright spot. For instance, y4 is
the distance from the center of the pattern to one of
the m = 4 bright spots on the screen. If the angle is
small (say, 10˚ or less), we can use the approximation
sinθ ≈ tanθ. Using that assumption, derive an
expression for ym in terms of d, m, λ, and L .
We have two equations to work with here, one for sinθ,
from above, and then one for tanθ, from the geometry
of right-angled triangles.
mλ
y
sin θ =
and tan θ = .
d
L
Figure 25.8: Spreading the dots out by
moving the screen farther away, starting
from the left-hand picture in Figure 25.7.
The small-angle approximation enables us to set these
two equations equal to one another. Doing that and
solving for y gives:
mλ L
.
d
(Eq. 25.5: The distance from the center of the
pattern to the m’th bright spot)
ym =
Step 4 – What would you do if you wanted to predict
the position of a particular bright spot on the screen,
but you could not use the small-angle approximation?
If we could not use the small-angle approximation, we
could first use the sinθ equation to find θ, and then
take the tangent of that angle when we were using
the tanθ equation to find y.
Figure 25.9: The geometry of the pattern,
based on a right-angled triangle.
Essential Question 25.2: A beam of light made up
of three wavelengths, 660 nm (red light), 530 nm
(green light), and 400 nm (violet light) is incident
on a diffraction grating that has a spacing of
d = 1300 nm. The first order spectrum, consisting of
a violet line, a green line, and a red line, produced
by the grating is shown in Figure 25.10. What are
the colors of the other three beams (1 – 3) that
Figure 25.10: The first-order (m = 1) spectrum for
come from the grating?
the situation of Essential Question 25.2.
Chapter 25 – Interference and Diffraction
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